Measurements on two essentially monodisperse fraction of a linear polymer, A and B, yield molecular weights of 100,000 and 400,000, respectively. Mixture 1 is prepared from one part by weight A and two parts by weight B. Mixture 2 contains two parts by weight of A and one of B. Determine the weight-average and number-average molecular weight average of mixtures 1 and 2.

weight average = fraction of A X MW of A + fraction of B x MW of B

Mixture 1

1 / 3 x 100000 + 2/3 x 400000 = 900000 / 3 = 300000

Mixture 2

2/3 x 100000 + 1/3 x 400000 = 600000 / 3 = 200000

Number average

Mixture 1

(1 / 100000 x 100000 + 2 / 400000 x 400000) / (1/100000 + 2/400000)

= 3 x 400000 / 6

= 200000

Mixture 2

(2/100000 x 100000 + 1/400000 x 400000) / (2/100000 + 1 / 400000)

3 x 400000 / 9

= (4/3) x 100000